SUMMARY: Fimbriae were found in 125 of 154 non-motile Klebsiella strains examined by electron-microscope in serial aerobic broth cultures. Fimbriate strains occurred in each of the capsule serotypes 1–72 and mostly showed the biochemical reactions of saprophytic . The fimbriae were clearly distinguishable from the capsules and occurred also in non-capsulate mutants. Most fimbriate strains showed evidence of varying reversibly between a fimbriate and a non-fimbriate phase. The proportion of bacilli with fimbriae was greatest in broth cultures and was decreased, though never eliminated, by serial cultivation on nutrient agar. All 29 permanently non-fimbriate strains belonged to serotypes 1–6 and showed biochemical reactions common in pathogenic and strains (anaerogenic, methyl-red positive, non-citrate utilizing or non-lactose fermenting). They differed from the fimbriate strains in growing less abundantly and usually in failing to form a pellicle on broth.

All the fimbriate strains, but none of the non-fimbriate, showed one or other or both of two kinds of adhesive property: one attributed to an ‘MS adhesin’, susceptible to inhibition by D-mannose and associated with a thick variety of fimbriae; the other due to an ‘MR adhesin’, resistant to mannose and associated with thinner fimbriae. Bacilli with MS adhesin rapidly adhered to red blood cells of the guinea-pig and other animals, except the ox, to leucocytes and to intestinal epithelial cells, including those of ox, to cells, to the mycelium of and other moulds, and to plant root-hairs. Bacilli with only the MR adhesin did not adhere to untreated red cells, leucocytes or ‘smooth’ yeasts, but adhered rapidly to ox and other red cells treated with tannic acid, red-cell stromata heated at 70° or 100°, fungus mycelium and plant root-hairs. Bacilli of many fimbriate strains and one non-capsulate non-fimbriate strain adhered to glass and cellulose.


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